Among various nonvolatile memories which hold data even after being powered off, a ferroelectric random access memory (FeRAM) including a ferroelectric capacitor writes data at high speed and may operate at a low voltage.
A ferroelectric capacitor includes a capacitor dielectric film made of a ferroelectric substance. When a voltage is applied between a bottom electrode and a top electrode, a polarization occurs in the capacitor dielectric film, and data is stored with the polarization direction of the capacitor dielectric film set to “0” or “1.”
Although ferroelectric capacitors of various structures have been proposed to improve characteristics of the FeRAM, they still have room for improvement.
For example, there have been proposed a method for forming a Lead Zirconium Titanate (PZT) film as a capacitor dielectric film, and then forming a top electrode containing RuO2 on the PZT film. However, in this method, ruthenium in the top electrode diffuses into the PZT film. As a result, a paraelectric layer in which the ferroelectric substance has changed into a paraelectric substance is formed in the PZT film near an interface between the PZT film and the top electrode. This reduces the thickness of the portion of the PZT film having a ferroelectric characteristic.
Moreover, the presence of the paraelectric layer causes a problem that a voltage applied to the ferroelectric capacitor is not applied efficiently to the PZT film due to the absorption of the voltage by the paraelectric layer.
Meanwhile, there has also been proposed a method in which a PZT film is formed as a capacitor dielectric film, an SRO (SrRuO3) film is formed thereon, and then a top electrode is formed on the SRO film. According to this method, the oxygen deficiency in the interface between the PZT film and the top electrode is offset by the SRO film.
However, since the SRO film is highly hygroscopic and absorbs moist in the air, the moist may reduce the PZT film and cause deterioration in the ferroelectric characteristic of the PZT film. Particularly in this method, the substrate is exposed to the air for a period after the formation of the SRO film until the formation of the top electrode, whereby a severe deterioration occurs in the PZT film due to the moisture absorption of the SRO film.
[Patent Literature 1] Japanese Laid-open Patent Publication No. 10-173142
[Patent Literature 2] Japanese Laid-open Patent Publication No. 2008-270596
[Patent Literature 3] Japanese Laid-open Patent Publication No. 2006-302975
[Patent Literature 4] Japanese Laid-open Patent Publication No. 2009-94200